Pitting corrosion
The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel that is more resistant to attack. The pitting corrosion resistance can be assessed using the Pitting Resistance Equivalent Number calculated from the alloy content.

Crevice corrosion
Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices, it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion-resistant grade.

General corrosion
Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature, and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and Sulphuric acid at some concentrations are particularly aggressive towards stainless steel.

Stress corrosion cracking (SCC)
This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. Typical applications where SCC can occur are hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.

Intergranular corrosion
This is now quite a rare form of corrosion. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitisation and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur. It is avoided by choosing a low carbon grade the so-called ‘L’ grades or by using a steel with Titanium or Niobium which preferentially combines with Carbon.

Galvanic corrosion
If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. This is rather like a battery and can accelerate corrosion of the less ‘noble’ metal. It can avoided by separating the metals with a non-metallic insulator such as rubber.

Stainless steel has satisfactory corrosion resistance in various industrial applications. Based on the experience of stainless steel applications, in addition to the mechanical failure of stainless steel, the corrosion form of stainless steel is mainly manifested as severe corrosion or local corrosion (that is, stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue, and crevice corrosion).

Stress corrosion cracking
The failure of stainless steel under stress due to crack expansion in a corrosive environment is called stress corrosion cracking. It has a typical brittle fracture morphology, but sometimes it also occurs in stainless steel with higher toughness. The necessary conditions for stress corrosion cracking to occur are tensile stress (including metal residual stress or external stress) and the presence of a specific corrosive medium. The generation and expansion of cracks are perpendicular to the direction of tensile stress. The stress value that can cause stress corrosion cracking should be much smaller than the stress value required for the material to break when there is no corrosive medium.
In metal microscopic, cracks that pass through grains are also called transgranular cracks, and cracks that extend along grain boundaries are called intergranular cracks. When stress corrosion cracking extends to a certain depth (here, the stress on the cross-section of the material under load reaches its fracture stress in the air), the material breaks as a normal crack. Therefore, the cross-section of the parts that fail due to stress corrosion cracking will contain the characteristic area of stress corrosion cracking and the "dimple" area associated with the aggregation of micro defects.

Pitting corrosion
This is also a form of local corrosion. If it is not handled well in the early stage, it is easy to make the stainless steel really corroded. If the protection work is done well in the early stage, it will not have much impact on the stainless steel plate surface.

Intergranular corrosion
The grain boundary is a disordered boundary between grains with different crystallographic orientations, so it is also the place where various solute elements in stainless steel are more likely to segregate or metal compounds precipitate. Therefore, it is normal that the grain boundary may be corroded first in some corrosive media. This type of corrosion is also called intergranular corrosion, and most metals and alloys may experience intergranular corrosion in certain corrosive media.

Crevice corrosion
This is also a form of localized corrosion, which may occur in crevices where the solution is stagnant or in shielded surfaces. Such crevices are generally formed between stainless steels or at the joints between stainless steel and non-metals, for example, in contact with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms.

General corrosion
General corrosion mainly describes the phenomenon of corrosion in a relatively uniform manner on the entire stainless steel surface. When general corrosion occurs, the stainless steel will gradually become thinner due to corrosion until it eventually fails due to corrosion. In strong acid and alkali environments, stainless steel may suffer from general corrosion. However, there is no need to worry too much about the failure caused by general corrosion, because this type of corrosion is usually easy to find.

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